Pulse oximetry involves the non-invasive monitoring of oxygen saturation level in blood-profused tissue indicative of certain vascular conditions. Pulse oximetry is typically used to measure various blood flow characteristics including, but not limited to, the blood-oxygen saturation of hemoglobin in arterial blood, the volume of individual blood pulsations supplying the tissue, and the rate of blood pulsations corresponding to each heartbeat of a patient. Measurement of these characteristics has been accomplished by use of a non-invasive sensor which passes light through a portion of the patient's tissue where blood perfuses the tissue, and photoelectrically senses the absorption of light in such tissue. The amount of light absorbed is then used to calculate the amount of blood constituent being measured. Oxygen saturation may be calculated using some form of the classical absorption equation know as Beer's law. The light passed through the tissue is typically selected to be of one or more wavelengths that are absorbed by the blood in an amount representative of the amount of the blood constituent present in the blood. The amount of transmitted light passed through the tissue will vary in accordance with the changing amount of blood constituent in the tissue and the related light absorption. For measuring blood oxygen level, such sensors have been provided with light sources and photodetectors that are adapted to operate at two or more different wavelengths, in accordance with known techniques for measuring blood oxygen saturation.
Known pulse oximetry sensors include an optical element which uses a pair of light emitting diodes (LEDs) to direct light through blood-perfused tissue, with a photodetector receiving light which has not been absorbed by the tissue. Accurate pulse oximeter measurements require relatively stable positioning of the sensor on an appendage, as well as proper alignment between the light source and light detector.
Accurate measurement of oxygen saturation levels are predicated upon optical sensing in the presence of arterial blood flow. A finger provides a convenient access to a body part through which light will readily pass. Other body appendages may also be used, e.g., toes and ears. Local vascular flow in a finger is dependent on several factors which affect the supply of blood. Blood flow may be affected by centrally mediated vasoconstriction, which must be alleviated by managing the perceived central causes. Peripheral constriction via external compression, however, can be induced by local causes. One such cause of local vasocompression is the pressure exerted by the sensor on the finger.
Many currently available pulse oximetry finger sensors have a hard shell which is maintained upon the finger tip by spring action. Since excess pressure on the finger can distort or eliminate the pulsation in the blood supply to the finger, these springs are intentionally relatively weak. The result of this compromise is that the spring-held sensors readily fall off the finger. Resilient polymer sensors are also known, such as disclosed in US Patent Publication No. 20060106294, incorporated by reference herein and assigned to Nonin Medical, Inc., the assignee of the present application. One limitation of these types of sensors has been user discomfort, particularly during extended periods of sensor use.
Many known non-disposable oximeter sensors are relatively bulky and exhibit a relatively high inertia of the housing relative to the finger. This results in a susceptibility to disturbance between the sensor and the finger surface as the patient's hand is moved. This relative motion manifests itself as motion artifacts in the detected signal. Motion artifacts, for example caused by tension on the lead wire, are especially problematic for pulse oximeter systems.
Pulse oximeter sensors are used in a number of applications where they are susceptible to being disturbed or displaced entirely from the appendage. Many oximeter finger sensors locate the lead wire from the sensor over a central portion of a patient's finger. When the patient flexes or curls his finger, the lead wire is often pulled against the sensor causing the light elements to be displaced.